The use of Doppler filtering for the elimination of echoes from stationary reflectors is well known in MTI-type radars. Such filtering requires the emission of outgoing pulses of identical carrier frequency by the radar antenna during successive range sweeps occurring while the antenna beam scans an outlying object, the number of those sweeps being therefore determined by the effective width of the radar beam generally defined by the -3 dB limits. Thus, the separation of clutter from moving targets is easy as long as the radar operates with a fixed carrier frequency.
In the presence of hostile ECM, however, fixed-frequency operation is susceptible to interference from unfriendly sources. Such interference is also not safely eliminated by so-called frequency diversity in which a predetermined number of carrier frequencies are recurrently used in a certain pattern, as with emission of a first burst (i.e. a succession of a certain number of sweeps) of one frequency followed by a second burst of another frequency, and so on. In such a case the filtering is carried out with echoes from pulses of the same burst.
A third technique, known as frequency agility, involves a random switching from one carrier frequency to another in consecutive sweeps in order to prevent ECM locking onto the frequency used. This mode of operation, while providing maximum safety from interference, is, however, incompatible with Doppler filtering as conventionally practiced.